The objective of this proposed research is to determine the cause of and the reasons for the partial degradation of T4 DNA into acid- soluble form, which we have found to be initiated almost immediately after infection of E. coli and to rise rapidly to a plateau within 5 min thereafter. The degradation is distinct from that due to superinfecting breakdown as it can be observed at very low m.o.i. (less than or equal to 0.05) and in endonuclease-minus cells that do not support superinfection breakdown. It is also observed in cells infected in the presence of chloramphenicol. The amount degraded is equivalent to 2-4% of the phage DNA. One obvious inference to be drawn is that the degradation could be a host-induced enzymatic digestion of the two molecular end-regions of the phage T4 DNA genome that promotes chromosome circularization in a manner Thomas demonstrated by his in vitro work on the same phage DNA. The research proposed here thus may be directly relevant to the current assumption that circularization is essential for normal extensive replication of linear phage chromosomes bearing terminally redundant sequences of genes. The following proposed series of experiments are designed to characterize the degradation: 1) test the kinetics and the extent of degradation in various host cells permissive to growth of T4 and whose deficiencies in specific nucleases and repair enzymes are known; 2) test the effects of chloramphenicol, rifampycin, cyanide and 3) of specific mutations, e.g. genes 32,43,46 and 47, in the phage on the kinetics and level of degradation; 4) look for a correlation between the number of radiochemical lesions in the DNA of irradiated phage and the rate and extent of degradation during early infection; 5) the degraded products will be analyzed chromatographically and their patterns compared to those arising from using different mutant cells mentioned above. We expect that the results obtained from these series of tests will help to identify the enzyme that causes the degradation and to decide if the phenomenon is relevant to phage chromosome circularization.